The man-cannon is a catapult that will launch you to several pre-determined locations on the map. In most maps, it'll launch you to a high point on the map - for example, on Urban Ultimatum, it will launch you to either of the towers on the map. This is a convenient way to get a strike force onto a guarded area. There are two disadvantages to using it. One: when you're getting ready to launch, you're very visible and very vulnerable. Two: after you've launched, you scream through the air and, again, are very visible. Essentially, there is no way to use the Man-Cannon and not have people know you're coming. Prepare for a hard fight as soon as you land.